At start-up of a continuous annealing furnace for the annealing of a steel strip which was once open to the air or in the case when the furnace allows the entry of air into the atmosphere therein, in order to decrease the concentrations of water and oxygen in the furnace, a conventional method that is widely performed is to raise the furnace temperature in order to vaporize the water in the furnace and, almost at the same time, to supply a non-oxidizing gas, for example, an inert gas as a purging gas to replace the atmosphere in the furnace while evacuating the gas in the furnace simultaneously, thereby purging the atmosphere in the furnace with the non-oxidizing gas.
However, such conventional methods require a long time to decrease the concentrations of water and oxygen in the furnace atmosphere to prescribed levels suited for steady operation. Thus, the discontinuation of operation during such a time drastically lowers productivity.
Further, in such fields as automobiles, home electric appliances and building materials, there have recently been increasing demands for high-tensile strength steel (high tensile steel) capable of contributing to enhancements such as of weight reduction of structures. In this high tensile technology, it is presented that the addition of silicon to steel possibly allows for manufacturing of high-tensile strength steel strips with good hole expandability, and further, the addition of silicon and aluminum facilitates the formation of retained γ, indicating the possibility that steel strips with good ductility may be produced.
However, high-strength cold rolled steel strips containing easily oxidizable elements such as silicon and manganese have a problem in that these easily oxidizable elements are concentrated at the surface of the steel strips during annealing to form oxides such as of silicon and manganese, deteriorating appearance or chemical conversion property such as phosphatability.
In the case of hot dip galvanized steel strips, the presence of easily oxidizable elements such as silicon and manganese in the steel strips causes a problem that these easily oxidizable elements are concentrated at the surface of the steel strips during annealing to form oxides such as of silicon and manganese, and such oxides impair coating properties to cause the occurrence of bare-spot defects or to decrease the alloying speed during an alloying treatment after the coating process. In particular, silicon is highly detrimental to coating properties and alloying treatments because a SiO2 film formed on the surface of a steel strip markedly lowers the wettability of the steel strip with respect to a hot dip coating metal and also because a SiO2 film serves as a barrier during an alloying treatment to inhibit the interdiffusion between the base iron and the coating metal.
A possible approach to preventing such problems is to control the oxygen potential in the annealing atmosphere.
To increase the oxygen potential, for example, Patent Literature 1 discloses a method in which the dew point in a latter half of a heating zone and in a soaking zone is controlled to a high dew point of −30° C. or above. This technique is expected to achieve effects to some degree and has an advantage that a high dew point may be controlled easily on the industrial scale. However, the technique is defective in that it does not allow for efficient production of some types of steel that do not favor being processed in a high-dew point atmosphere (for example, Ti-containing IF steel) because an annealing atmosphere once brought to a high dew point requires a very long time to become one having a low dew point. In this technique, further, the furnace atmosphere is oxidative and, unless controlled appropriately, causes a problem of pick-up defects due to the attachment of oxides to rolls in the furnace as well as a problem of damage to the furnace walls.
Lowering the oxygen potential is another possible approach. However, because such elements as silicon and manganese are highly prone to oxidation, it has been considered that there will be great difficulties in stably maintaining the atmosphere with a low dew point of −40° C. or below at which excellent suppression is possible of the oxidation of elements such as silicon and manganese, in a large continuous annealing furnace such as one disposed in a CGL (continuous hot dip galvanization line)-CAL (continuous annealing line) system.
For example, Patent Literature 2 and Patent Literature 3 disclose techniques for efficiently obtaining a low-dew point annealing atmosphere. These techniques reside in relatively small, single-pass vertical furnaces and are not designed to be applied to multi-pass vertical furnaces such as CGL and CAL systems. Thus, it is highly probable that these techniques will fail to decrease the dew point efficiently in a multi-pass vertical furnace.
In some multi-pass vertical furnaces having a heating zone and a soaking zone, the heating zone and the soaking zone are physically separated from each other by a partition wall disposed therebetween except for traveling routes for a steel strip. Other such furnaces have no partition wall between the heating zone and the soaking zone, namely, the heating zone and the soaking zone are not physically separated from each other. As compared with the case where a partition wall is present, the absence of a partition wall between the heating zone and the soaking zone allows the gas in the furnace to flow with a higher degree of freedom and with higher complexity. Thus, difficulties are frequently encountered in decreasing the dew point in the entirety of the furnace.
Patent Literature
PTL 1: International Publication No. WO 2007/043273
PTL 2: Japanese Patent No. 2567140
PTL 3: Japanese Patent No. 2567130